Security: The script could wipe out the hard drive or worse. But with a script at least the user can verify the intentions by looking at the source. Even current add-ons (layers) are often .exe installers giving the user no way of identifying evil code.

To unload a script's assembly from memory completely, WW needs a restarted (there are workarounds (appdomains))

With no nailed down limited interface into WW the script could stop working on a new version of WW.